This invention concerns the measurement of pressure within a body cavity. More specifically, this invention pertains to a catheter adapted to be used for the measurement of intrauterine pressure during labor.
Fetal monitoring is a standard procedure for monitoring the condition of a fetus during childbirth. In most cases, fetal heart rate and intrauterine pressure are measured and separately plotted on a strip-chart recorder. By examining the plotted curves, the onset of certain distress conditions can be detected so that appropriate remedial action can be taken earlier than would otherwise be possible.
To measure intrauterine pressure a catheter may be inserted into the uterus and filled with liquid so that the force of the intrauterine contractions can be transmitted through the uterine fluids and the liquid in the catheter, to a pressure-measuring device such as a strain gauge or the like.
In prior art applications of intrauterine catheters of the type described, a cumbersome procedure is required to fill the catheter with liquid and then couple the catheter to the strain gauge to complete a liquid path or column from the uterus to the gauge. Conventionally, the catheter is inserted through a relatively rigid guide tube which is curved to conform to the vaginal canal. The catheter is pushed through the guide tube until the uterine end of the catheter is located correctly within the uterus. The liquid is injected by means of a syringe and suitable adaptor into the catheter. It is then necessary to slide the guide tube off the catheter which requires removal of the adaptor and syringe through which the liquid was injected into the catheter.
After the guide tube has been removed, the catheter is coupled by means of the adaptor to a three-way stopcock, with the other inlets of the stopcock being connected, respectively, to the luer fitting of the strain gauge and the syringe. This enables the bleeding of air which is likely to enter the system during insertion of the catheter into the uterus. The bleeding operation requires first that water be again injected into the catheter. The catheter is then closed by the stopcock and the syringe used to inject water into the strain gauge dome until all air is removed. Next, the syringe is removed to obtain a "zero" setting by opening the strain gauge (after the air bubbles have been removed) to atmosphere. The "off" lever on the three-way stopcock is then rotated so that the catheter is coupled directly to the strain gauge.
During insertion, known intrauterine catheters of the type described may be "plugged" by materials within the vaginal canal. If this happens, pressure measurements are no longer meaningful and the catheter must therefore be cleaned which invariably requires that the system be again bled as described above.
Even if a sterile liquid is used, when the liquid injection and air bleeding procedures take place in an insterile environment, use of these standard catheters can be accompanied by substantial risk of infection.
One approach to solving the aforementioned problems is disclosed in U.S. patent application Ser. No. 658,821 (now U.S. Pat. No. 4,136,681), which is assigned to the assignee of this application. U.S. Pat. No. 4,136,681 discloses a prefilled catheter sealed on one or both ends by a limp membrane through which uterine pressure is transmitted to a strain gauge or other suitable measuring device. The catheter is packaged in a longitudinally slit guide tube and sealed by a film to prevent the sterile fluid in the catheter from escaping. The film must be punctured to permit entry of the catheter into the uterus and must be torn along its length to permit removal of the guide tube from around the catheter. The principal drawback of the device is that it is relatively difficult and expensive to manufacture.
The main object of the invention is to provide an intrauterine catheter which is easier to use than the catheters currently in use. A more specific object of the invention is to provide a low cost, disposable catheter for measuring uterine pressure which does not have to be bled of air at the time of use.